A piezoelectric material changes shape when subjected to an electric field. With the appropriate structure design, this change of shape can be translated into a displacement. In positioning actuators, such as piezoelectric motors, this displacement is harvested into linear or rotary motion. When the displacement is at ultrasonic frequencies, the piezo motors are defined as piezoelectric ultrasonic motors. Piezoelectric ultrasonic motors are frequently driven at resonance or semi resonance to take advantage of the amplification of the displacement due to the resonance itself. Piezoelectric ultrasonic resonant motors are used in a variety of motion control applications that require for example precise stepping, high acceleration, high velocity, small size, high force, zero-power hold, and no magnetic fields.
Positioning actuators, such as piezoelectric ultrasonic motors, may be driven using full bridge or half bridge drive circuitry. In the case of an ultrasonic motor, the motor vibrates mechanically at a frequency dictated by the drive circuitry and, if the amplitude of displacement is sufficient, the motor will generate external motion.
Positioning actuators have a multitude of practical applications, including for positioning a camera lens, such as the lens of a smartphone, camera phone, or other camera device. Positioning actuators used, for example in AutoFocus systems for mobile phone camera modules, require fast settling times (on the order of 20 ms or less) and low audible noise (on the order of 35 dB or less). Unfortunately, while fast settling times are dependent on how quickly the position control system can move a lens into position, the acoustic audible emissions of positioning actuators driven by the control system are increased as the positioning actuators are driven at higher velocity.
Different strategies have been developed to set the velocity of external motion for positioning actuators, such as ultrasonic motors, with the aim of reducing power and audible acoustic noise. Given these existing strategies to set the velocity, it also is desirable to adjust the output velocity of the positioning actuator. Such adjustments may be done using an open loop control system or a closed loop control system.
An open loop controller, also defined as a non feedback controller, is a controller that computes its input in the system based only on the current state of the system and the model of the system that the controller has. It is relatively cheap since it does not require feedback sensors but can be very imprecise with nonlinear systems.
With a closed loop controller, also defined as feedback controller, the controller determines the input in the system based on a difference between feedback from a sensory system and a target set point. Closed loop control systems are more expensive due to the need for a sensory system, but they are more precise and can be adaptive.
In the context of a positioning actuator that is used, for example, to drive a focusing element, such as a lens into a target position in less then twenty milliseconds, a very efficient closed loop control system is required. Unfortunately, positioning actuators may have high variability in terms of their control factors. For example, the distance a moveable object can be moved as the result of a certain number of input driver pulses (pulses per unit distance) can vary one order of magnitude or more from one positioning actuator to another and is dependent on many factors. These factors can include the driving voltage, the driving waveform, the location of the positioning actuator contact with the moveable element relative to the total range of travel, the surface properties of the friction parts, and tolerances of the parts and assembly. This variability makes many positioning actuators, such as friction based ultrasonic motors, unsuitable for closed loop control using a proportional-integral-derivative controller (PID controller) as is often common for other types of motors. Therefore, efficient control schemes to enable fast operation with suitable noise reduction remain elusive for positioning actuator systems, such as ultrasonic motors.